Preheat oven to 450 degrees F (230 degrees C). Prick potatoes several times with a fork and place onto a baking sheet.
Bake in the preheated oven until potatoes are easily pierced with a fork, 50 minutes to 1 hour. When potatoes are cool enough to handle, scoop potato flesh out into a bowl and mash lightly with a fork.
Cook and stir SPAM(R) Classic in a skillet over medium heat until browned (optional).
Melt butter in a large pot over medium-low heat; stir in flour to form a smooth roux. Cook and stir roux until well combined and bubbling; stir in milk. Season with salt and pepper.
Cook milk mixture, stirring continuously, until it thickens, about 5 minutes; stir in potatoes and SPAM(R) Classic and cook until heated through.
Serve in bowls and garnish each with about 1/4 cup of Cheddar cheese.